dumpstate: improve performance

Two changes:

1. There were some seemingly-unnecessary fsyncs scattered throughout
dumpstate. Remove those.

2. Profiling revealed that a significant amount of dumpstate's CPU
overhead was spent copying/destroying PTEs around fork/exec. Switch to
vfork() to avoid the PTE copies.

Test: bug reports work
Bug: 215574756
Change-Id: Ib202fd5c8d1f37548ee53bd64eaf4e114098a062